SCF at the Center of Ethylene Signaling

Ethylene is a gaseous plant hormone that is involved in many developmental processes, as well as in responses to pathogens and stress. Two groups report that the ethylene pathway involves regulated degradation of the transcription factor EIN3 by the F-box proteins EBF1 and EBF2. F-box proteins specifically recruit target proteins to an SCF ubiquitin ligase for degradation by the proteasome. Guo and Ecker show that EIN3, the transcription factor stimulated as a result of ethylene signaling, increases in abundance in response to ethylene. The increase in EIN3 required perception of ethylene by the ethylene receptor and functional EIN2, a transmembrane protein involved in ethylene signaling. Stabilization of EIN3 did not require protein synthesis and could be achieved by pharmacologic inhibition of the proteasome. Guo and Ecker and Potuschak et al. used yeast two-hybrid screens to identify an interaction between EIN3 and the F-box proteins EBF1 and EBF2. EBF1 and EBF2 were also determined to bind other proteins that comprise the SCF complex [(SKP1/cullin/F-box protein). SCFs consist of four subunits: a cullin, SKP1 (S-phase kinase-associated protein 1), a RING finger protein (RBX1/HRT1/ROC1), and an F-box protein], confirming their role as part of this ubiquitin ligase complex. Genetic analysis demonstrated that EIN3 was stabilized in ebf1 and ebf2 mutants. These mutant plants also demonstrated the "triple response" typical of increased ethylene signaling, which was not observed if ein3 was also mutated. Overexpression of EBF1 and EBF2 led to decreased EIN3, and the plants were not responsive to ethylene. These results show that ethylene, like auxin, jasmonate, and salicylic acid, controls transcription by regulatory protein degradation. However, unlike other plant hormones that stimulate degradation of negative regulators, ethylene inhibits degradation of a positive regulator (EIN3).